Here is from the FAQ on the HDS web site... pay attention to paragraph's 3-5.
What are lumens, spec sheet lumens and out-the-front lumens?
The Lumen is the international unit of luminous flux, which is a measure of the total "amount" of visible light emitted by a source. Luminous flux differs from power (radiant flux) in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements indicate the total power of all electromagnetic waves emitted, independent of the eye's ability to perceive it.
Your eyes are not uniformly sensitive to all colors of light. Your eyes are most sensitive to green light and least sensitive to violet and red light. The lumen takes the relative sensitivity of your eyes into consideration. The spectral content of light is multiplied by the sensitivity curve of your eye to create a final result. Thus, a milliwatt of green light (555nm) counts much higher in lumens than a milliwatt of violet light (415nm) or a milliwatt of red light (630nm).
Measuring lumens is difficult as it requires specialized equipment - namely an integrating sphere with calibrated spectrometer and special software. We start the measurement process by calibrating the spectrometer using a NIST (National Institute of Standards and Technology) traceable light source with a known spectral content. This calibration step is needed to account for measurement errors in the spectrometer. Then the light being measured must be broken up into the individual wavelengths and each wavelength must be accurately measured using the calibration data to correct any measurement errors. This results in a power spectrum. The power spectrum is then multiplied by the lumen sensitivity curve to generate a lumen spectrum. Finally, the lumen spectrum is integrated to generate a final calibrated lumen value.
A calibrated spectrometer can also be used to calculate the correlated color temperature (CCT), the color rendering index (CRI), CIE values and many other interesting numbers. We use a calibrated spectrometer for all of our LED and flashlight evaluations.
A light meter is not a calibrated spectrometer and cannot be used for accurate light measurement. A light meter is a single sensor behind a filter and can only take a single measurement. The filter and sensitivity of the sensor only roughly approximate the lumen spectral response. In fact, most light meters come with instructions on how to adjust the meter reading depending on the type of light source you have. Remember, light meters were designed for photographic and building illumination use so if you are off by half an f-stop (1.4x), it is no big deal because both film and the human eye are
logarithmic.
The lumen is directly related to two other units of light measurement: lux and candela. A lux is the measure of illuminance, which means how much light is falling on a surface. The candela is the measure of luminous intensity, which means how much light is being emitted in a particular direction. These units of measure have the following relationships. A lux is one lumen spread over one square meter. A candela is one lumen spread across the solid angle of one steradian. The area at one meter distance from the point of a cone that occupies one steradian is one square meter. Thus one lumen is equal to one lux which is equal to one candela for one steradian at one meter from the source.
LEDs are tested after manufacture and sorted into bins. One of the bin categories has to do with how many lumens the
LED will generate under a well defined set of conditions. For example, one manufacture places
LEDs that generate from 87.4 lumens to 113.6 lumens at 350mA into their U bin and
LEDs that generate from 113.6 lumens to 147.7 lumens into their V bin. The manufacturer allows a production tolerance of +/-10% for the measuring equipment. This means that the output of
LEDswithin one bin can vary over a 40% range.
It is common for a flashlight manufacturer to improve their flashlight's apparent output specification by claiming that they are generating the average bin lumens stated on the
LED manufacturer's specification sheet. Or they may state the output specification as "up to" the maximum bin lumens stated on the
LED manufacturer's specification sheet. Both of these are deceptive practices. This is referred to as spec sheet lumens.
An example will help you understand the deception of spec sheet lumens. Suppose we have an
LEDfrom a bin with a typical 40% spread - 180 lumens to 300 lumens. The average will be 240 lumens. The "up to" will be 300 lumens. And that is before we have taken losses from the optical system into consideration. In the end, you can end up with a 200 lumen flashlight being marketed as a 300 lumen flashlight. But wait, the deception gets worse.
The
LED's spec sheet lumens are typically measured under ideal conditions at 25°C - before the
LEDhas a chance to heat up. Once the
LED operates at full power for a short period it will heat up and loose at least 20% of it's lumen output. So if the
LED was from the bottom of the bin, you may find the
LED is generating less than 50% of the claimed lumens under real conditions. The "up to" 300 lumens flashlight may only be generating 150 lumens - at the
LED. Once you take the optical system losses into account, you may be getting 35 to 45% of the claimed lumens out the front of the flashlight. The "up to" 300 lumens flashlight may actually be emitting a mere 110 lumens out the front of the flashlight.
A better way to rate flashlights is to measure the light output after it has passed through the optical system and once the
LED has had a chance to warm up. This is sometimes referred to as out-the-front (OTF) lumens. This is the method prescribed by the
ANSI FL-1 standard. Unfortunately, the
ANSI FL-1 standard allows you to perform the testing on 3 "representative" lights, which does not take care of the unit-to-unit variations between individual
LEDs within the same bin. Two flashlights with the same lumen rating can still produce easily visible differences in output and be compliant.
HDS Systems is the only manufacturer to go the final step and calibrate each flashlight. We measure the output of each flashlight after it is completely assembled and adjust the output to produce the specified lumen output. This method allows the
LED to warm up to operating temperature as part of the measurement process so the flashlight's true lumen output can be measured and adjusted under representative operating conditions. Thus, our 250 lumen flashlight can be counted on to actually produce 250 lumens.